The present invention relates to an apparatus for discharging or removing a developing agent from a magnetic brush developing device for use in an electrophotographic copying or printing machine.
In the magnetic brush developing device, use is made of a two component developing agent consisting of carriers made of magnetic material and toners made of thermoplastic resin of low melting point having mixed with dye or pigment such as carbon black. Such a two component developing agent is stirred and mixed in the developing device and the toners are charged due to friction between the toners and carriers to a polarity opposite to that of an electrostatic charge image on a charge retentive member such as a photosensitive drum.
Usually the magnetic brush developing device comprises a housing having an opening at a position which is opposed to the charge retentive member, a sleeve made of non-magnetic material is arranged rotatably in the housing at such a position that a part of the sleeve surface exposes the outside of the housing through the opening and a magnet assembly arranged fixedly in a space within the sleeve. In such a device, when the sleeve is rotated in a given direction, the developing agent attracted onto the sleeve surface is transported to the opening and is brought into contact with a surface of the charge retentive member through the opening. Also, in such a developing device since the toners are consumed each time the developing operation is effected, the developing device is provided with a hopper for containing toners and a mechanism for supplementing a given amount of toners from the hopper so as to keep always a toner concentration of the developing agent to a desired value. On the contrary, the magnetic carriers are not consumed in the developing operation, but are repeatedly used. However, the carriers are fatigued mechanically and physically due to an agitation, secular variation, etc., and thus should be exchanged by new carriers after the developing device has been used for a predetermined time period or a predetermined number of developing operation have been effected. In order to exchange the used and fatigued carriers by new ones, the fatigued carriers must be first discharged or removed from the developing device together with toner particles and then a new developing agent consisting of new carriers and toners must be introduced into the developing device.
Heretofore, there have been proposed various kinds of apparatus for discharging the fatigued developing agent from the magnetic brush developing device. For instance, in Japanese Utility Model Publication No. 50,533/76 there is disclosed a discharging apparatus in which a box having a scraping edge is detachably secured in a developing device in such a manner that the scraping edge is made in contact with a rotatably arranged sleeve surface. When the sleeve is rotated, the developing agent attracted onto the sleeve surface is fed to the scraping edge and is scraped off the sleeve. The developing agent thus scraped is collected in the box. Then, the box is removed from the developing device. In such an apparatus since a magnet assembly of the magnetic brush developing device is fixedly arranged during the discharging operation and thus the developing agent is fed only due to a frictional force between the developing agent and sleeve surface, the agent is fed or transported to the scraping edge of box only with a very small force. Further, in this known apparatus during the discharging operation the sleeve is rotated in a direction which is same as that during the normal developing operation. As is well known the magnetic brush developing device comprises a doctor blade for limiting an amount of developing agent attracted onto the sleeve surface. In the known discharging apparatus since the scraping edge is arranged at a downstream side with respect to the doctor blade (when viewing in the rotating direction of sleeve), an amount of the developing agent fed to the scraping edge is limited by the doctor blade. Therefore, an efficiency of the discharging operation is very low and thus the discharging operation is a quite time consuming one. Further, when an amount of developing agent remained in the developing device becomes small near the end of the discharging operation, the agent attracted onto the sleeve near the poles of the magnet does not move together with the sleeve, but is remained near the poles. This results in that a part of the developing agent is always remained in the developing device without being discharged.
Further, in the known discharging apparatus, the box for receiving the developing agent has to be installed in a space within a housing of the developing device and thus the developing agent is liable to spread outside the developing device and moreover operator's hands might be stained with the developing agent.